Prohibition aims at reducing (1)/ traffic accidents many of which (2)/ is caused by drunkenness. (3)/ No error (4)

Answer

3

Explanation

are will replace is because - many (Pronoun) is the antecedent and Pronouns like few, many, several, both, all, some always take the Plural form.
More over, the sentence is in Passive Voice
The structure in Passive Voice is as follows :
Sub. + Aux.V. + Main V. (Past Participle) + by + Agent (doer)
Hence, are causedby drunkennessis theright usage